Permanganate-oxidizable C (POXC) and mineralizable C (as determined by short-term aerobic incubation of rewetted soil) are measures of active organic matter that may provide early indication of soil C stabilization and mineralization processes. To date, the relationship between these two promising active organic matter tests has not been comprehensively evaluated, and little is known about their functional role in the soil ecosystem. Here, we examined the relationship between POXC and mineralizable C across a wide range of soil types, management histories, and geographic locations across the United states (13 studies, 76 total sites; n = 1071) and the ability of POXC and mineralizable C to predict crop yield and total aboveground biomass. Results from this comparative analysis showed that POXC and mineralizable C are related (r 2 = 0.15-0.80) but that the relationship was differentially influenced by management practices. Overall, POXC better reflected practices that promote organic matter accumulation or stabilization and therefore can be a useful indicator of long-term soil C sequestration. Conversely, mineralizable C better reflected practices that promote organic matter mineralization and therefore can be a useful indicator of short-term soil nutrient availability. Our results also show that both mineralizable C and POXC were better predictors of corn (Zea mays L.) grain yield, aboveground biomass, and tomato (solanum lycopersicum L.) fruit yield than other soil C fractions evaluated here. Thus, the integrated use of POXC and mineralizable C can provide a complementary framework to assess the relative dynamics of soil C stabilization and nutrient mineralization functions in agroecosystems. O f the three pools that constitute soil organic matter (SOM), the active or labile pool is comprised of rapidly cycling organic material that mostly turns over in a shorter time frame (days to a few years) relative to the intermediate (a few years to decades) and stable (decades to centuries) pools (Cambardella and Elliott, 1992;Gregorich et al., 1994;Parton et al., 1987;Wander, 2004 Core Ideas• POXC and mineralizable C were evaluated across diverse agroecosystems.• The two are related but differentially influenced by management practices.• POXC better reflected sOM stabilizing practices.• Mineralizable C reflected sOM mineralizing practices.• Both predicted agronomic performance better than other soil C fractions.
Inter‐laboratory variability for mineralizable C is greater than for other commercial soil tests. Water content and direction of rewetting both affect values of mineralizable C. As a soil health indicator, mineralizable C should have a standardized protocol. Analytical variability of mineralizable C is highly affected by soil type. Mineralizable C, or C that is respired upon the rewetting of dried soil, is a common metric of soil health, but the metric still lacks a widely accepted and standardized protocol. A standardized protocol is an essential first step in quality control needed for a robust soil test. Here we examined numerous sources of laboratory variability associated with mineralizable C, with the overall goal of understanding the influence of each source on final values. Mineralizable C had twofold to 20‐fold greater inter‐laboratory variability than other commonly used soil tests, leading to a high degree of uncertainty associated with the interpretation of results. Procedural differences—such as sieve size and the method of rewetting—significantly influenced measurements of mineralizable C and underscore the need for the development of a standardized and universally adopted protocol. Capillary rewetting consistently suppressed mineralizable C relative to rewetting with a specific amount of water and is therefore not a recommended approach. However, the sensitivity of mineralizable C to changes in management did not differ among incubation intervals of 6, 24, and 72 h. While these procedural effects may influence inter‐laboratory variability, there was also a considerable amount of analytical variability associated with mineralizable C measurements within a laboratory that is highly dependent on soil type.
Nitrogenous fertilizers have nearly doubled global grain yields, but have also increased losses of reactive N to the environment. Current public investments to improve soil health seek to balance productivity and environmental considerations. However, data integrating soil biological health and crop N response to date is insufficient to reliably drive conservation policy and inform management. Here we used multilevel structural equation modeling and N fertilizer rate trials to show that biologically healthier soils produce greater corn yields per unit of fertilizer. We found the effect of soil biological health on corn yield was 18% the magnitude of N fertilization, Moreover, we found this effect was consistent for edaphic and climatic conditions representative of 52% of the rainfed acreage in the Corn Belt (as determined using technological extrapolation domains). While N fertilization also plays a role in building or maintaining soil biological health, soil biological health metrics offer limited a priori information on a site's responsiveness to N fertilizer applications. Thus, increases in soil biological health can increase corn yields for a given unit of N fertilizer, but cannot completely replace mineral N fertilization in these systems. Our results illustrate the potential for gains in productivity through investment in soil biological health, independent of increases in mineral N fertilizer use. Since the Green Revolution, nitrogenous mineral fertilizers have helped to nearly double global grain crop yields 1. While this represents monumental gains in crop productivity, an estimated 41 to 50% of the N fertilizer applied to corn (Zea mays L.) globally since of the Green Revolution has been lost to the environment 2 , resulting in multifarious negative environmental effects 3-5. Many strategies to address N losses from cropping systems are centered on the management of N fertilizer (e.g., "4Rs" of fertilizer management 6), but neglect the role of soil 4,5 biology in supplying plant N which often supplies over 50% plant N uptake in a growing season 7-9. The framework of soil health seeks to highlight the critically important 10 , albeit inherently complex and uncertain 11,12 , role of soil biology in agroecosystems. Ultimately, soil health seeks to integrate soil biology with the historically emphasized chemical and physical soil components 13,14. Soil health has been widely embraced by farmers, researchers, and private industry alike 15-18. Additionally, soil health has also accrued broad legislative support in the form of nearly a dozen states incentivizing improved soil health as well as a Soil Health division within USDA 19. Buy-in from these stakeholders represents a nexus of several key sources of information growers use in making nutrient management decisions 20-22 , as well as a demonstrated financial commitment. While soil health has broad conceptual support, there is a dearth of empirical evidence connecting soil biological health measurements to vital soil functions or desired outcomes 23. Althou...
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